System and method for solar cell array communication

ABSTRACT

In one implementation, a method for a solar cell array is provided, the method includes emitting a communication message from the solar cell array by reverse biasing the solar cell array so as to cause at least a portion of the solar array to emit a detectable amount of radiation corresponding to the communication message. In one embodiment a solar cell array circuit is provided including a solar string comprising a plurality of solar cells coupled together, a charge storage device coupled to a power bus, and a bidirectional boost-buck converter having a first and second pair of MOSFETs connected in series between positive and negative rails of the power bus with an inductor coupled from between the first and second paired MOSFETs to a charging output of the solar string.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.17/606,444, filed Oct. 25, 2021, entitled SYSTEM AND METHOD FOR SOLARCELL ARRAY COMMUNICATION, by Lotfy et al., herein incorporated byreference in its entirety, which is a National Stage Application ofPCT/US2020/029979, filed Apr. 25, 2020, entitled SYSTEM AND METHOD FORSOLAR CELL ARRAY COMMUNICATION, by Lotfy et al., herein incorporated byreference in its entirety, which claims the benefit of the followingapplications, which are all herein incorporated by reference in theirentireties:

U.S. Provisional Application No. 62/893,756, filed Aug. 29, 2019, byNader Lotfy et al., entitled SYSTEM AND METHOD FOR SOLAR CELL ARRAYCOMMUNICATION, which is herein incorporated by reference in itsentirety; and

U.S. Provisional Application No. 62/838,937, filed Apr. 25, 2019, byNader Lotfy, entitled SYSTEM AND METHOD FOR SOLAR ARRAY DIAGNOSTICS.

BACKGROUND

In certain situations, it may not be possible, practical, or desirableto use conventional communication systems to communicate. In someinstances, for example conventional communications are not possible dueto equipment failure. In other instances, for example, it may not bedesirable to allow conventional communications to be intercepted. In yetother situations, using conventional communications could make theperson transmitting detectable.

What is needed is a system and method that can be used in suchsituations, as well as others.

SUMMARY

In one possible implementation, a method for a solar cell array isprovided, the method includes emitting a communication message from thesolar cell array by reverse biasing the solar cell array so as to causeat least a portion of the solar array to emit a detectable amount ofradiation corresponding to the communication message.

In one possible embodiment, a solar cell array circuit is providedincluding a solar string comprising a plurality of solar cells coupledtogether, a charge storage device coupled to a power bus, and abidirectional boost-buck converter having a first and second pair ofMOSFETs connected in series between positive and negative rails of thepower bus with an inductor coupled from between the first and secondpaired MOSFETs to a charging output of the solar string.

In one possible implementation, a method is provided for communicating amessage with a solar cell array in a high altitude long enduranceaircraft. This particular embodiment includes displaying a message on asolar cell array on the high altitude long endurance aircraft, detectingthe message using a satellite, and relaying the detected message fromthe satellite to a platform below the high altitude long enduranceaircraft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified schematic of a bidirectional circuit for asolar cell array.

FIG. 2A is a timing diagram showing operation of the bi-directionalcircuit in both the charging mode.

FIG. 2B is a timing diagram showing operation of the bi-directionalcircuit in the display mode.

FIG. 3 is a simplified diagram of a circuit having a solar array.

FIG. 4 is a simplified diagram of an improved circuit for a solar array.

FIG. 5 is a simplified diagram of an improved circuit for a solar array.

FIG. 6 is a simplified diagram of a string circuit.

FIG. 7 is a simplified diagram of an improved solar string circuit.

FIG. 8 is a plot showing an example V-I curve of voltage versus currentfor a typical solar array system.

FIG. 9 is a plot illustrating an example V-I curve of voltage versuscurrent for a solar array system for high performance solar cellutilized in high altitude long endurance aircraft implementations.

DESCRIPTION

In one possible implementation in accordance with the present invention,it is useful to communicate using a solar array as a communicationdevice. To achieve this, selected parts, or all of the solar array maybe made to emit radiation instead of its normal function of absorbingsunlight and converting it to electrical power. Thus, in accordance withsome embodiments of the present invention, it is possible to bias solarcells to cause them to emit radiation, during times when they are notabsorbing it and cause them to emit radiation for communicationpurposes.

In some implementations, the emitted radiation may be in one or more ofthe visible light, the infrared, or other spectrum, depending on thesolar cell device characteristics. Thus, the solar cells would beselected to perform both normal solar energy conversion, and for itsdesired communication spectrum. Further, it is envisioned that the solarcells could be tailored to emit certain frequencies for suchcommunication purposes, when not being utilized for solar collectionpurposes.

Typically, solar cells are used in an array to absorb radiation, andgenerate electrical power therefrom for use in a system, or for storagefor later use. Since solar power use is becoming more widespread.Implementations, in accordance with the present invention provide a newapplication for solar cells, as communication devices. This can beapplied not only to conventional fixed terrestrial applications, butalso to other applications of solar cells.

In a particular implementation, solar arrays are utilized in UnmannedAerial Vehicles (UAVs), aircraft with no onboard pilot, which may flyautonomously, or be remotely piloted. In high altitude long enduranceaircraft, a solar cell array may be used as source of power, which thebatteries, engines, and other aircraft systems use.

In high altitude long endurance aircraft, for example, the solar cellarray is typically positioned on upper surfaces of the aircraft, such ason the upper surfaces of the wings, or on the upper surface of thefuselage, or both. High altitude long endurance aircraft are typicallylight weight aircraft with large wingspans, sometimes a long as 100meters or more. Further, they can have high lift wings and can be madeto fly relatively slow. The large wingspan of the high altitude longendurance aircraft covered with a solar array provides large surfacearea for visual viewing during communications. An advantage of using thesolar array for communication is that because the array is on the topsurface of the wing, the communication is directed upward and notviewable from terrestrial or lower aircraft. It is, however, viewable bysatellite detectors, such as optical, infrared, or other frequencies.Thus, the solar array may be utilized for directional communications.

In other applications, solar arrays may be portable, or even wearable.Solar cells, or panels, can be made relatively flexible, so can beaffixed to textiles, such as articles of clothing, or to other wearablearticles. The wearable solar panel is then be used to charge portablebatteries, or battery equipped/powered equipment or devices while beingpart of a garment or accessory. In such applications, the solar array isfurther utilized for nearby or distance communications.

The solar array may be configured to operate to radiate in a displaymode for communication in the near visible light, visible light, orother detectable spectrum to convey messages. For example, in a wearablearticle of clothing, the wearer could cause messages to emit in visiblelight from the article of clothing to communicate line of sightmessages. Similarly, in solar assisted/powered automobiles, solar cells,or arrays, could be used to indicated position or to communicate driverintention (as blinker or other indicator/communication light), or toflash messages.

In the case of an aircraft, the direction of communications may beadjusted by adjusting the aircraft orientation, which changes theorientation of the solar array. Similarly, the orientation ofsatellites, or of actuatable terrestrial solar panels, could be adjustedto change the communication direction. In the case of portable devices,including wearable solar arrays, the orientation of the portable device,or the wearable device may be adjusted to direct communications.

FIG. 1 shows a simplified schematic of a bidirectional circuit 100 for asolar cell array. The solar cells may be arranged into solar strings 110having several solar cells 105 coupled together. In this embodiment, thesolar string 110 is coupled to the power bus 195 via a two quadrantbidirectional boost-buck converter 170. Thus, the solar string 110 iscoupled to the power bus 195 via inductor 171, such as for example,about 300 to 600 micro Henries, between a MOSFET 173 and a MOSFET 174.The MOSFET 173 and the MOSFET 174 are connected in series between thepositive and negative rails of the power bus 195. The control inputgates of the MOSFETs 173 and 174 are connected to a microcontroller formodulating the control inputs so as to cause the converter 170 to supplycurrent from the solar string 110 to the bus 195 in charging mode, or todraw current from the bus 195 to drive the solar string 110 to emitradiation in display mode. In the charging configuration, the MOSFET 174is modulated, such as pulse width modulated, while MOSFET 173 is eithermodulated or just used as a diode to supply power to the bus 195 forcharging. In display configuration, MOSFET 173 is modulated, whileMOSFET 174 is open, or modulated.

The solar strings 110 may include several nearby solar panels, whichcould be linear, square, rectangular, or other geometric configurationof solar panels. Or, the solar panels in a string may be spaced so as toenable display of patterns, characters, letter, numbers, symbols,images, machine readable type linear or matrix bar coding, or the like,to allow communication, either by direct, coded, or even aestheticcommunication. The display may be time variant or encoded messages, suchas emission duration encoded, or frequency encoded messages, or evenintensity encoded, or other known encoding method.

In one simplified example, the solar cell could emit messages by opticalMorse code. Or, in another simplified example, a bar coded message, orcharacter message could be statically displayed, or dynamically scrolledacross the wingspan of a high altitude long duration unmanned aircraft.In one possible scenario, the displayed message could be in response toreceived messages when the aircraft is unable to otherwise transmit vianormal communication channels, i.e. where the aircraft's transmitter isnot functioning, or where general open transmission of the message isnot desirable.

In another possible application, such as wearable, other mobile, or evenfixed terrestrial applications, the solar cells could be selected toemit only an infrared spectrum signal, which could be detected only withinfrared detection means, such as infrared/night vision goggles,portable infrared detectors, such as binoculars or other handheldinfrared detection devices, and/or by aircraft forward or side lookinginfrared detectors, for example.

In still another useful application, a stranded, lost, or person notable to move from their position, could utilize the display mode on thesolar device to transmit an SOS, another signal or message, or to merelyilluminate, such as in the visual, or IR spectrum for identification orrescue at night using optical, or IR detection means, i.e. binoculars orIR imaging devices, such as IR goggles. As such, for use incommunication the display emits a detectable amount of radiation suchthat it may be observed either directly unaided, or with the aid of anelectronic detector capable of detecting in the displayed spectrum andconverting it to an directly observable radiation, or by converting itto information, i.e. such as night or regular vision collection and/oramplification, conversion from one or more spectrum to another,detection and conversion for an information display, such as a handhelddevice display, projection display, a display monitor, a readable ortextual display, a location identification display, a mapping display,or the like.

Some embodiments can display information generated by an aircraft wherethe inputs are detected by the aircraft onboard sensors. This displaycould be coordinated to display vehicle conditions, such as vehiclespeed.

FIGS. 2A & 2B are timing diagrams 200 a and 200 b showing operation ofthe bi-directional buck-boost converter in both the charging mode and inthe display mode. The MOSFET 173 and the MOSFET 174 are operated byadjusting the duty cycle and pulse width of the MOSFETS 173 and 174 in acomplementary fashion to provide the charge mode and the display mode.In the charge mode, the duty cycle of MOSFET 174 is increased withrespect to the MOSFET 173. In the display mode, the MOSFET 173 isincreased with respect to the MOSFET 174.

The timing diagrams 200 a and 200 b show example timing plots 200 a and200 b illustrate how pulse width modulation of MOSFET 173 and the MOSFET174, respectively, will produce/allow the positive or negative currentI_(L) flow generated from of the solar string 110 during charging, orinto the solar string 110 from the power bus 195 during display mode.Timing plot 200 a shows how the positive voltage V_(L) pulse train 270p, with respect to ground, applied to the battery 190 side of theinductor 171 produces a positive current I_(L), while the other plot 200b produces negative current I_(L). For each plot 200 a and 200 b, thereis a voltage V_(L) pulse train 270 p and 270 n above its resultantcurrent I_(L) plot 273 and 274, respectively. The MOSFETS 173 and 174pulsed to cause voltage V_(L) square wave pulses 270 p or 270 n toproduce positive 273 or negative 274 sawtooth current I_(L). The widervoltage pulses 270 p contribute a positive current I_(L), while thenarrower pulses 270 n contribute a negative current I_(L). This willallow either normal charge mode charging operation of the solar string105, or display mode display operation of the solar string 105.

FIG. 3 is a simplified diagram of a circuit 300 having a solar array.Typically, several independent solar arrays or strings 310 a, 310 b, and310 c connected in parallel to form a channel 320. In some embodiments,a string may have a number of replaceable solar panels, for example.Each of the strings 310 a, 310 b, and 310 c is provided with a serialconnected blocking diode 315 a, 315 b, and 315 c. The blocking diode 315a, 315 b, and 315 c are provided in case one of the strings 310 a, 310b, and 310 c shorts, to prevent one of the strings 310 a, 310 b, or 310c from causing shorting, or other failure, in the other non-shortedstrings 310 a, 310 b, or 310 c. Although the blocking diodes add anamount of loss to the system, they are required to reduce failures ofassociated the solar cells within the array, or other components withinthe system.

The blocking diodes are typically located within a power tracker 380,which also has a boost stage 385 DC/DC converter. The boost stage 385decouples the supplied voltage and the current from the high voltagepower bus 395, i.e. 270 V-400 V, that is connected to the battery 390,and is configured so that proper voltage is supplied on the high voltagepower bus regardless of the voltage and current supplied by the solararray. In one embodiment, the power tracker is a maximum power pointtracker or MPPT controller configured to boost voltage from the solararray to the output and to adjust a boost ratio to get the maximum powerfrom the solar array. Examples of MPPT controllers include Outback®FLEXmax 60/80 MPPT, Xantrex® MPPT Solar Charge Controller, and Blue Sky®Solar Charge Controller. Generally speaking, the MPPT controller isconfigured to maximize the available power going into the battery fromthe solar array. This is important in various high altitude longendurance aircraft applications where the maximum voltage is a functionof the temperature and illumination of the solar array, both of whichmay vary throughout the day.

With the circuit of FIG. 3 , however, display mode is not possiblebecause the diodes 315 a, 315 b, and 315 c prevent reverse biasing ofthe solar strings 310 a, 310 b, or 310 c. The embodiments of FIG. 4 ,and FIG. 5 , discussed below, permit display mode because it is possibleto reverse bias the solar cells 105.

FIG. 4 is a simplified diagram of an improved circuit 400 for a solararray. In this embodiment, in place of the blocking diodes 315 a, 315 b,or 315 c (FIG. 3 ), a solar string control MOSFET 415 is used in thechannel 410. This configuration is more efficient than the circuit ofFIG. 3 because it eliminates the voltage drop loss across a diode 315 a,315 b, or 316 c when the solar string control MOSFET 415 is turned on.This embodiment, however, when the solar string control MOSFET 415 isturned on, it does not inherently provide isolate between parallelconnected solar strings in the event there is a short in a solar string410.

As such, in this embodiment, the voltage drop across the solar stringcontrol MOSFET 415 is detected and monitored so as to determine themagnitude and direction of the current in the solar string 410 todetermine whether another solar string (not shown) has shorted. If it isdetermined, based on the monitoring of several parallel connectedstrings, that one of the solar strings has shorted, the solar stringcontrol MOSFET in line with the shorted solar string is opened so as notto damage the other parallel connected solar strings. With the solarstring control MOSFET 415 open, it performs as a diode as in FIG. 3 , toblock current through the shorted solar string.

Thus, the embodiment of FIG. 4 provides a more efficient power transfer,but requires monitoring of the current in the solar string 410 todetermine if a short has occurred in the solar string 410, or in any ofthe associated strings. Typically, the monitoring and control of thesolar string control MOSFET 415 is conducted by a power tracker, orother associated electronics. The power tracker includes the solarstring control MOSFET 415 and the boost stage 485, which supplies powerto the power bus 495.

FIG. 5 is a simplified diagram of an improved circuit 500 for a solararray. In this embodiment, in place of the blocking diodes 315 a, 315 b,and 315 c (FIG. 3 ), MOSFET switches 515 a, 515 b, and 515 c areutilized having back-to-back MOSFETS devices 515 a ₁ and 515 a ₂. TheMOSFET switch allows each string 510 a, 510 b, or 510 c in the channel520 to be individually completely disable or open, even though theoutput of multiple strings go into a single power stage, such as a booststage 585 DC/DC converter, which supplies current to the power bus 595for charging the battery 590. The boost converter 585 controls theMOSFET switches 510 a, 510 b, or 510 c and utilizes the detected stringvoltages to determine the health of the strings 510 a, 510 b, or 510 calong with all the other strings (not show) and channels (not show) overthe entire solar array. An advantage of this it that it allows thesystem to target each string 510 a, 510 b, or 510 c and perform a lot ofdiagnostics in flight, such as short circuit current, open circuitvoltage, on a per string basis.

Replacing the protection diodes with MOSFET switches is undesirable in aterrestrial solar system because it increases the cost of the system.Using the MOSFET switches, however, is very desirable in high altitudelong endurance aircraft, where it is significant and important toextract the maximum energy from the solar array. MOSFET switches can beselected to have lower power loss across the switch as compared to aprotection diode. As, such it increases the efficiency of the chargingsystem, and also allows you to predict (through trend analysis) anddetect failures much easier. Imminent failures can be predicted andaction taken before a failure becomes critical. This is important inhigh altitude long duration aircraft, so as to enable avoidance of acritical failure that could otherwise lead to a power off, or even acrash landing. Since strings can be individually tested, it providesmuch more “visibility” into the functionality and health of the solararray in flight.

Moreover, each string can be tested in flight to determine the optimalpower output for each string individually according to its V-I and poweroutput characteristic. As such, the string characteristics can be testedover time to determine the health of the string. This is particularlyimportant during long duration flights, and during high altitudeflights, so that the need for maintenance and/or remedial measures, suchas solar panel replacement, can be anticipated and made when convenient.

In some embodiments, each string includes a number of solar panelsgrouped generally spanwise along the wing of the aircraft, for examplefour, five, or six smaller solar panels are grouped per string. Thesolar panels are grouped in this way so that the solar panels in astring experience similar environmental and operational conditionstogether. For example, the solar panels at the leading edge of the wingmay be grouped together in a string, while solar panels near thetrailing edge of the wing may be grouped together, possibly with one ormore strings also extending spanwise or laterally along the span of thewing, between the leading and trailing edge strings.

In high altitude applications, the grouping of the solar panels intostrings is significant. This is because the temperature can vary greatlyfrom the leading to the trailing edge of the wing. Further, theorientation of the aircraft with respect to the sun, in elevation,azimuth, rotation, etc., as well as having a greater curvature fromleading to trailing edges of the wing, can further exaggerate thetemperature differential. In high altitude long endurance solar poweredaircraft, the temperature can range across the wing from −60 degreesCelsius on the leading edge to +60 degrees Celsius on the trailing edgeof the wing. As such, grouping solar panels into strings combined withbeing able to individually switch on or off individual strings based onthe performance of a string allows for more efficient solar powergeneration.

Turning to FIG. 6 , shown is a simplified diagram of a string circuit600. In FIG. 6 , the string 600 typically has a bypass diode 645 inparallel with two or more solar cells 605 a and 605 b. The bypass diode645 allows the other solar cells 605 m to supply current around thesolar cells 305 a and 605 b when one or more of the solar cells becomesnon-functional or an open circuit, such as by being cracked, or broken.

With reference to FIG. 6 , in a further embodiment, the bypass diode 615is replaced by a bypass MOSFET switch 760 as shown in the simplifieddiagram of a solar string circuit 700 of FIG. 7 . Such an embodiment,allows further efficiency over the diode when a solar cell 605 a, 605 b,. . . or 605 m (FIG. 6 ) fails, due to the lower power loss associatedwith a MOSFET switch 760 as compared to the bypass diode 645.Furthermore, it allows for closer monitoring and predictive analysis ofindividual, or a small group of solar cells for better predictiveanalysis of the string. As the maximum allowable current for the string710 is restricted by the restriction of the lowest individual solarcell, being able to bypass only one or several individual solar cells705 a and 705 b, or others, can be used to optimize power output of thestring 710.

One advantage of various embodiments over string circuits using blockingdiodes, is that the blocking diodes can contribute a loss of about 0.7%,whereas MOSFET switches can reduce those losses. Though discussed abovewith respect to a MOSFET switch, other comparable type switch, i.e lightweight, low loss switch, could be utilized in other embodiments.Furthermore, although shown in FIG. 5 with only three solar strings in achannel for illustration purposes, embodiments may contain two or morestrings, and multiple channels.

Various embodiments enable, or expand the capability to run in-flightdiagnostics. In high altitude long endurance solar power aircraft,factors such as turbulence, frequent and extreme thermal, motorvibrations can increase the possibility of failure. Various embodiments,provide performance tracking over time, with trend analysis, and canenable eminent failure detection, more flexible scheduling ofservice/periodic maintenance, and avoidance of lack of airborne networkcapability/coverage or surveillance capability/coverage in the coveragearea. This is particularly important if the high altitude long enduranceaircraft is being used as a cellular repeater or for other networkcommunications in area that would otherwise be without coverage shouldthe platform be missing from the network.

FIG. 8 is a plot 800 showing an example V-I curve 810 of voltage versuscurrent for a typical solar array system. The power curve 820 for thesolar array system is superimposed on the plot 800. It is desirable toextract the maximum power from the solar array system. As such, it isdesirable to operate along the V-I curve 810 where the power for thesystem is at its peak.

FIG. 9 is a plot 900 showing an example V-I curve 210 of voltage versuscurrent for a solar array system for high performance solar cellutilized in high altitude long endurance aircraft implementations. Invarious high performance solar cell implementations which may beutilized in high altitude long endurance aircraft, the V-I curve 910 andthe power curve (not shown) have a very steep slope as they approach themaximum current. Thus, the optimum operating point of the system lieswith a narrow operating range. If the current is too great by even aslight amount, the voltage goes to zero or short circuits very easily.For example, the difference between optimum power output and shortcircuit can be as low as 100 mA of current per channel 120 (FIG. 1 ).Depending on solar cell and channel configuration, this could be evenlower in some embodiments, such as 75 mA, 50 mA, 25 mA, or less. Toavoid this while achieving the highest power output, a voltage loop isutilized to monitor voltage while determining the peak power operatingpoint, as well as monitoring the current and power. This is because thechange in voltage is much bigger than either the power or the currentnear this point.

Thus, to find the optimum operating point of the system, the current isregulated, while monitoring the voltage as well as the power. Thecommanded current is varied by power point tracker circuitry, whilemonitoring power. Additionally, the voltage is also monitored todetermine when the power output is maximized because the rate of changeof the voltage is greater than the rate of change of the power at nearthe maximum power output operating point.

To achieve the most efficiency in some embodiments, the voltage ismonitored at a faster rate than the current and power. In someembodiments, the voltage may be monitored ten time faster than thecurrent or power. For example, the current and/or power may be monitoredat 10 times a second, while the voltage is monitored at 100 time asecond.

This enables various embodiments to extract the most amount of solarpower from the solar panels in high altitude long endurance aircraftapplications, without drawing too much current and sending the voltageto zero, thereby shorting the solar cell.

It is important to note that factors such as the solar panel temperatureand the amount of solar exposure can shift the maximum power operatingpoint. In high altitude long endurance solar powered aircraft, thesefactors are more significant because the temperature range across thesolar cells is more extreme, as discussed further below, and the shadingor shadowing is typically experienced more frequently and to a greaterdegree. Thus, monitoring and adjusting the operating point is especiallyimportant in high altitude long endurance solar powered aircraft. FIG. 9depicts example V-I curves for hotter temperature 910 and coldertemperature 911 for higher solar intensity or bright solar exposure 910,and lower solar intensity or shadowed solar exposure 912.

It is worthy to note that any reference to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment may beincluded in an embodiment, if desired. The appearances of the phrase “inone embodiment” in various places in the specification are notnecessarily all referring to the same embodiment.

The illustrations and examples provided herein are for explanatorypurposes and are not intended to limit the scope of the appended claims.This disclosure is to be considered an exemplification of the principlesof the invention and is not intended to limit the spirit and scope ofthe invention and/or claims of the embodiment illustrated.

Those skilled in the art will make modifications to the invention forparticular applications of the invention.

The discussion included in this patent is intended to serve as a basicdescription. The reader should be aware that the specific discussion maynot explicitly describe all embodiments possible and alternatives areimplicit. Also, this discussion may not fully explain the generic natureof the invention and may not explicitly show how each feature or elementcan actually be representative or equivalent elements. Again, these areimplicitly included in this disclosure. Where the invention is describedin device-oriented terminology, each element of the device implicitlyperforms a function. It should also be understood that a variety ofchanges may be made without departing from the essence of the invention.Such changes are also implicitly included in the description. Thesechanges still fall within the scope of this invention.

Further, each of the various elements of the invention and claims mayalso be achieved in a variety of manners. This disclosure should beunderstood to encompass each such variation, be it a variation of anyapparatus embodiment, a method embodiment, or even merely a variation ofany element of these. Particularly, it should be understood that as thedisclosure relates to elements of the invention, the words for eachelement may be expressed by equivalent apparatus terms even if only thefunction or result is the same. Such equivalent, broader, or even moregeneric terms should be considered to be encompassed in the descriptionof each element or action. Such terms can be substituted where desiredto make explicit the implicitly broad coverage to which this inventionis entitled. It should be understood that all actions may be expressedas a means for taking that action or as an element which causes thataction. Similarly, each physical element disclosed should be understoodto encompass a disclosure of the action which that physical elementfacilitates. Such changes and alternative terms are to be understood tobe explicitly included in the description.

Having described this invention in connection with a number ofembodiments, modification will now certainly suggest itself to thoseskilled in the art. The example embodiments herein are not intended tobe limiting, various configurations and combinations of features arepossible. As such, the invention is not limited to the disclosedembodiments, except as required by the appended claims.

What is claimed is:
 1. A solar cell array circuit comprising: a) a solarstring comprising a plurality of solar cells coupled together; b) acharge storage device coupled to a power bus; and c) a bidirectionalboost-buck converter comprising: (i) first and second paired MOSFETsconnected in series between positive and negative rails of the powerbus; and (ii) an inductor coupled from between the first and secondpaired MOSFETs to a charging output of the solar string.
 2. The circuitof claim 1 comprising a MOSFET device connected in series between theinductor and the charging output of the solar string.
 3. The circuit ofclaim 1 further comprising a MOSFET switch being connected in seriesbetween the inductor and the charging output of the solar string.
 4. Thecircuit of claim 3, wherein the MOSFET switch comprise back-to-backcomplementary MOSFET devices.
 5. The circuit of claim 1, comprising aplurality of the solar strings configured so as to be capable ofemitting least one of: (1) patterns; (2) characters; (3) letters; (4)numbers; (5) symbols; (6) images; or (7) combination thereof.